1. Field of the Invention
This invention relates to current collectors/electrodes for batteries and methods for producing current collectors/electrodes.
2. Description of the Related Art
Since the very early days of commercially available lead-acid batteries in the late 19th Century, battery electrodes have been made from pasted plates. Such plates, called “current collectors,” commonly have a support base made of a porous matrix, such as a metal grid. Traditionally the grid is a lead alloy in which the holes are filled with an electro-active paste such as a mixture of red lead and 33% dilute sulfuric acid. The process of applying the paste to the matrix is referred to as “pasting”. The term “matrix”, as used herein, refers to the base structure of a current collector to which the electro-active paste is applied. Such a matrix may be characterized or classified, in part, on the basis of its specific surface area, normally expressed in the units m2/m3. The term “ground substance” and “matrix substrate” refer to the porous substance of which a matrix is made.
Recently attempts have been made to find non-metal matrix substrates that are more suitable for current collectors than lead grids. The goal has been to find a sturdy, light-weight, porous substrate that retains electro-active paste in the hostile environments and operating conditions of a wide range of battery applications. Kelley et al, U.S. Pat. No. 6,979,513 (“Kelley”), describe the use of carbon foam to form a battery current collector. Gyenge et al in U.S. Pat. No. 7,060,391 (“Gyenge”) teach the use of carbon foam deposited with a layer of lead-tin alloy in the construction of a current collector for a lead acid battery.
These devices can improve the utilization efficiency of positive active mass, and battery energy density. However, current collectors such as those of Kelley and Gyenge employ cast solid lead or lead alloy frames and/or lead connectors—also known as “lugs”—in order to improve electrical current distribution and structural integrity. These lead frames and/or lugs are relatively heavy, which negates any weight savings achievable from the use of carbon foam, and, consequently, there is basically no or negative gain in power density and little improvement in energy density. Similarly, the weight of present nickel metal hydride battery current collectors is relatively high.
An additional drawback is that carbon foam is fragile and lacks structural integrity, which complicates manufacturing processes such as battery pasting and battery assembling. For instance, whilst a carbon foam matrix is much lighter than metal-based matrices carbon foam current collectors of the Gyenge type must be thicker than a conventional lead grid as a result of the need to maintain the structural integrity and strength of the carbon foam. Consequently, the number of such thick current collectors that can be arranged in parallel and series internally is actually less than in a battery using conventional lead grids. This means that a lead acid battery employing the Gyenge type current collectors have lower power density than the conventional lead acid battery.
Those skilled in the art are also familiar with deficiencies of other types of current collectors. For example, metal-foil current collectors presently used in lithium ion batteries have at least two problems: 1) low volume cathode material loading, given that the battery has a very thin layer of cathode material applied onto the metal sheet current collector, limiting the battery capacity; and 2) high risk of thermal runaway due to poor electrical conductivity of the cathode material such as Li—CoO2, Li—MnO2, and Li—FePO4. Thermal runaway may occur, for example, when a microscopic impurity such as copper or nickel is mixed inside the cathode material, which can converge on one spot, leading to a substantial electrical short and development of a sizable current between the positive and negative plates.
From the foregoing discussion it is apparent that a need exists for a matrix that is made of ultra light material and yet has sufficient rigidity and strength to serve as the base for a current collector. Accordingly, the present invention provides a novel and non-obvious multiply-conductive matrix (MCM) for a current collector that results in improved battery power density, energy density, and electrical conductivity as well as enhanced battery safety.